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><H1
><A
NAME="SQL-LISTEN"
></A
>LISTEN</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN72899"
></A
><H2
>Name</H2
>LISTEN&nbsp;--&nbsp;listen for a notification</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN72904"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>LISTEN <TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
></PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72907"
></A
><H2
>Description</H2
><P
>   <TT
CLASS="COMMAND"
>LISTEN</TT
> registers the current session as a
   listener on the notification channel named <TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
>.
   If the current session is already registered as a listener for
   this notification channel, nothing is done.
  </P
><P
>   Whenever the command <TT
CLASS="COMMAND"
>NOTIFY <TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
></TT
> is invoked, either
   by this session or another one connected to the same database, all
   the sessions currently listening on that notification channel are
   notified, and each will in turn notify its connected client
   application.
  </P
><P
>   A session can be unregistered for a given notification channel with the
   <TT
CLASS="COMMAND"
>UNLISTEN</TT
> command.  A session's listen
   registrations are automatically cleared when the session ends.
  </P
><P
>   The method a client application must use to detect notification events depends on
   which <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> application programming interface it
   uses.  With the <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> library, the application issues
   <TT
CLASS="COMMAND"
>LISTEN</TT
> as an ordinary SQL command, and then must
   periodically call the function <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> to find out
   whether any notification events have been received.  Other interfaces such as
   <SPAN
CLASS="APPLICATION"
>libpgtcl</SPAN
> provide higher-level methods for handling notify events; indeed,
   with <SPAN
CLASS="APPLICATION"
>libpgtcl</SPAN
> the application programmer should not even issue
   <TT
CLASS="COMMAND"
>LISTEN</TT
> or <TT
CLASS="COMMAND"
>UNLISTEN</TT
> directly.  See the
   documentation for the interface you are using for more details.
  </P
><P
>   <A
HREF="sql-notify.html"
>NOTIFY</A
>
   contains a more extensive
   discussion of the use of <TT
CLASS="COMMAND"
>LISTEN</TT
> and
   <TT
CLASS="COMMAND"
>NOTIFY</TT
>.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72930"
></A
><H2
>Parameters</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>channel</I
></TT
></DT
><DD
><P
>      Name of a notification channel (any identifier).
     </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72938"
></A
><H2
>Notes</H2
><P
>   <TT
CLASS="COMMAND"
>LISTEN</TT
> takes effect at transaction commit.
   If <TT
CLASS="COMMAND"
>LISTEN</TT
> or <TT
CLASS="COMMAND"
>UNLISTEN</TT
> is executed
   within a transaction that later rolls back, the set of notification
   channels being listened to is unchanged.
  </P
><P
>   A transaction that has executed <TT
CLASS="COMMAND"
>LISTEN</TT
> cannot be
   prepared for two-phase commit.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72946"
></A
><H2
>Examples</H2
><P
>   Configure and execute a listen/notify sequence from
   <SPAN
CLASS="APPLICATION"
>psql</SPAN
>:

</P><PRE
CLASS="PROGRAMLISTING"
>LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448.</PRE
><P></P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72951"
></A
><H2
>Compatibility</H2
><P
>   There is no <TT
CLASS="COMMAND"
>LISTEN</TT
> statement in the SQL
   standard.
  </P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN72955"
></A
><H2
>See Also</H2
><A
HREF="sql-notify.html"
>NOTIFY</A
>, <A
HREF="sql-unlisten.html"
>UNLISTEN</A
></DIV
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